The present invention relates to electromagnetically deflected cathode-ray-tubes and particularly to apparatus for correcting distortion of electron beams as caused by the magnetic field of an electromagnetic deflection yoke.
Electromagnetically deflected cathode-ray-tubes suffer from distortion of the spot written by the tube's electron beam or beams on the CRT screen as a result of nonuniformities in the magnetic deflection field. Thus, when a magnetic deflection yoke is energized to deflect an electron beam from its center position to a more peripheral position, the electrons in the electron beam may be deflected somewhat differently depending upon their position in the beam. As a result the electron beam becomes defocused despite the presence of focusing means in the electron gun that generates the beam. In one instance, as the beam is deflected radially outwardly from the center screen position, the spot tends to become elongated in a direction perpendicular to the direction of deflection. Thus, if the beam is deflected horizontally, to the right of center screen, the spot may tend to be elongated in a vertical direction. Although this astigmatism distortion is not sufficiently serious to prevent the use of electromagnetically deflected cathode-ray-tubes for many purposes, the distortion can be of considerable concern where finely detailed information is to be presented, e.g. on the screen of a computer terminal portraying a great deal of graphic and alphanumeric information. A raster scan may be employed in the case of CRTs used in these applications wherein the raster is composed of a large number of lines as compared with the conventional TV application.
A prior art apparatus for correcting the astigmatism produced in a single beam by an electromagnetic deflection yoke is described in Ray et al U.S. Pat. No. 3,961,223. This system employs a set of correction coils wound upon a separate annular magnetic core positioned around the neck of the cathode-ray-tube. Current is supplied to this set of coils, via amplifiers for converting correction voltages to coil currents. In a first pair of coils, a current is provided which is proportional to (Vx).sup.2 -(Vy).sup.2, where Vx and Vy are horizontal and vertical deflection signals, while a second set of coils is provided a current proportional to 2VxVy. Electromagnetic correction devices and the circuitry therefor tend to be expensive and cumbersome not only in respect to the cost of the equipment but also in regard to proper adjustment external to the cathode-ray-tube. Moreover, the correction achieved is somewhat approximate and does not take into account the considerable differences in the astigmatic distortion produced by different magnetic deflection yokes.
A similar problem is encountered in multi-beam tubes used for color television. For example, when utilizing a three beam in-line gun, a self-convergent deflection yoke characterized by a field with a third harmonic component insures convergence of the electron beams with horizontal deflection, i.e. deflection in the plane of the electron beam generating means. Although the result is electron beam convergence despite horizontal displacement of the electron beams, the effect of the field on the individual beam produces astigmatism distortion. Thus, vertical overfocusing occurs which can result in a vertically elongated spot or in a horizontally elongated spot as the result of added spherical aberration. Prior art electromagnetic correction is not possible without defeating the self-convergence desired for the three beams, and moreover electromagnetic correction is not able to correct for different degrees of astigmatism produced in the three beams as a result of their displaced positions. While some astigmatism correction has been provided in tubes employing in-line guns by shaping of beam-generating electrodes already present in the conventional gun, this attempt at correction is only partially effective and does not achieve correction dynamically. Other prior art astigmatism correction schemes are not capable of correcting differently distorted beams in an individual manner.